Abstract: Late Holocene Foraminiferal Biofacies from Baffin Bay, Texas: Preliminary Analysis of a 4000-Year Record of Paleosalinity

Laura Stewart, Martin B. Lagoe, E. William Behrens

Predominant hypersalinity in Baffin Bay, Texas, leads to the preservation of laminated sediments. These largely unbioturbated sediments permit the reconstruction of a high-resolution, late Holocene paleoenvironmental record. Piston cores from the central portion of Baffin Bay recover up to 5 m of this sediment. Based on previous dating of dolomite layers and sedimentation-rate estimates (ca. 1 mm/yr), these cores represent up to 4000 yr of deposition. Preliminary micropaleontological analysis of core L593-2 (4.9 m long) is based on 120 samples, with an average sample spacing of 3-4 cm. Foraminiferal assemblages are dominated by three major taxonomic groups: (1) Ammonia parkinsoniana; (2) Elphidium spp.; and (3) miliolids. Previous work on modern microfaunas in Texas mbayments shows that each of these groups may predominate under different environmental conditions. Miliolids dominate hypersaline environments whereas the other two groups will tolerate moderately brackish water. Accordingly, core samples dominated by miliolids are interpreted to represent the most hypersaline conditions. Samples dominated by Ammonia parkinsoniana represent the most brackish conditions encountered in the core (but >20 ppt). Dominance of assemblages varies dramatically within the core. Major biofacies trends include (1) no occurrence of either low-salinity agglutinated assemblages or open-shelf biofacies; (2) peaks in miliolid abundance define paleosalinity fluctuations on a scale of 10-30 cm in the upper 4.2 m of the core; and (3) miliolids are very rare or bsent from 4.2 to 4.9 m, suggesting reduced hypersalinity, but still restricted conditions. The core did not penetrate sediments deposited prior to restriction of Baffin Bay by Padre Island (ca. 5000 Ma). Three dolomite layers within the core (at 2.86 m, 4.34 m, and 4.71 m) occur in intervals with rare or no miliolids, arguing against a hypersaline setting for their formation. The type of paleoenvironmental record preserved in Baffin Bay has great significance for paleoclimatic studies of the late Holocene.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994